1. Field of the Invention
This invention relates to a hologram apparatus, and more particularly to a hologram apparatus which stores continuous information at high density and reconstructs information with high precision.
2. Description of the Prior Art
In general, apparatuses for storing and reconstructing continuous information should have a high storage density and a high reconstruction fidelity. Especially in an apparatus which employs voices or pictures as the continuous information, the demand is great because the quanitity of the voice or picture information is enormous. As one type of device to meet to such demand, a video disk is known. On the other hand, packaging devices or systems for pictures and sounds have hitherto widely used devices, such as a VTR (video tape recorder) and an EVR (electronic video recorder), which record the information on a tape. Unlike these devices, the video disk forms a relief in the surface of a disk (made of, for example, a resin) and stores the information by way of pattern of the relief. The disk of this type is greatly advantageous in that, since the operation of taking replicas from a master is possible by simple press techniques used in well-known sound record disks, a multiplicity of disks can be reproduced inexpensively.
The video disks are broadly classified into the bit-by-bit system and the holographic system. According to the former, information of pictures and voices is developed into time serial signals recorded spirally over substantially the entire area of the predetermined disk. In this case, the area on the disk surface occupied by a unit signal is as small as 1 .mu.M = 1 .mu.m. A serious technical problem is the manner in which the small areas are correctly traced to read out and reproduce the signals. In other words, it is a problem of tracking. On the other hand, a video disk according to a holographic system is shown, by way of example, in FIG. 1. Referring to the figure, information corresponding to one picture frame is represented by a single hologram 1. A plurality of the holograms 1 are arranged spirally on a disk 2. While rotating the disk 2, a laser beam 4 from a laser source 3 illuminates the holograms 1 in succession. Reconstructed images 5 from the holograms 1 are picked up by a photodetector, such as camera tube, 6. Electric signals from the photodetector 6 are reproduced by a television monitor 7 in a well-known manner. Shown at 2' is the rotary shaft of the disk 2. In such a construction, the size of the unit hologram is, for example, 1 mm in diameter. The tracking of the holograms by the laser beam involves no problem of precision. The apparatus illustrated in FIG. 1 represents therefore a preferred, simplified construction.
A technical problem in the case of recording the holograms on the disk stated above is how sound signals corresponding to one picture frame are associated with each hologram. In this respect, various expedients have heretofore been proposed. It is a real situation, however, that none of the expedients is effective. An example of the proposals is a system in which, in spirally arranging the holograms of the pictures, a slight spiral interstice is provided between the adjacent hologram arrays in the radial direction, to form sound record grooves in the interstice.
This system is illustrated in FIG. 2. In the figure, a spiral indicated by reference numeral 8 is the sound record groove. With such a system, however, picture information is recorded on the greater part of the entire disk surface, and hence, only a very small amount of voice information can be recorded when the area of the disk is that of an ordinary sound record. This is a grave restriction in practical use.